Avid recently started a support line for unsupported systems, if you can believe that. I heartily congratulate them for doing so!
Be that as it may, you have too much RAM (Windows doesn’t really see/use anything above 3GB’s as of WinXP Pro…that will change with Vista). I back Jon’s opinion that if you’re doing this for a hobby, try using what you’ve got. Keep in mind that there’s an excellent chance you will run into problems when you add Mojo, since Mojo doesn’t seem to like most homebuilt systems. (Not all of them, since I know a few folks who are running homebuilt computers with Mojo and absolutely no problems at all, though those people are extremely rare.) Why is a long story.
If you are using this computer to make money, I suggest simply buying a certified system, as it will alleviate most of your issues, give you peace of mind, and keep you up and running (which means more profitable). You can get incredible deals on certified systems on Ebay to get you started, then upgrade to an even more current system in a year or so.
Some thoughts for you to consider.